I guess you must have missed out on Streets of Rage, Final Fight, Double Dragon, Golden Axe, The Simpsons, TMNT, Bad Dudes, River City Ransom, X-Men, Punisher, Captain America, Battletoads, Captain Commando, Aliens vs. Predator, Cadillacs and Dinosaurs, Dungeons and Dragons, Knights of the Round, Spider Man Maximum Carnage, and Castle Crashers. Ya know, among a few others.

Co-op has been a part of this genre from the beginning, and is featured in the best beat 'em ups of all time. C'mon, son.